This invention relates to a method for coupling a semiconductor laser used as a source for optical signal transmission such as in an optical communication system and an optical measurement system, with an optical fiber.
A conventional device for coupling a semiconductor laser with an optical fiber is, for example as disclosed in the Japanese Laid-open Patent No. 57-211288, constituted as shown in FIG. 1.
The output light from a semiconductor laser 1 is focused on an input plane 5 of an optical fiber 3 through a coupling lens 2, and is led into a core 4 of the optical fiber 3. Generally, when a semiconductor laser is used as the source for an optical signal transmission system, part of the output light from the semiconductor laser is reflected from the coupling system, and when it returns to the semiconductor laser, it is known to cause problems, such as instability of the oscillation characteristic of the semiconductor laser and increase of noise. In particular, the reflection from the fiber input plane returns at a high percentage, and to solve this problem, the input plane 5 of the optical fiber 3 is inclined so as to prevent the light reflected by this plane from returning to the emission point of semiconductor laser 1.
In such an arrangement, however, as shown in FIG. 2, since the input plane 5 of the optical fiber 3 is oblique, the beam axis 7 of the input light in the optical fiber is inclined by an angle .phi.' with respect to the axis of the optical fiber, and the coupling efficiency is lowered accordingly. (This phenomenon is more obvious when a single-mode fiber is used than when a multimode fiber is used.) The angle .phi.' is determined, assuming the angle between a plane perpendicular to the axis of the optical fiber of the input plane of the optical fiber to be .phi.'and the refractive index of the optical fiber to be n.sub.1, from the relation according to Snell's law ##EQU1##